This invention relates to a surgical implant and more particularly, but not exclusively, to a cervical plate assembly for stabilising the cervical spine.
Plates are commonly used to connect two separated pieces of bone. These plates immobilise the broken bone pieces while the bone pieces fuse together. Cervical plates are commonly used for anterior spine stabilisation to assist fusion in cases of degenerative disease, tumours, fractures and partial or total vertebrectomy. The plates are fixed to the bone by screws. It is known to provide the screws with expansion cones to expand the tip of the screw after insertion of the latter in the bone. This results in secure fixation. However, it is often difficult.in young patients or in good quality bone to expand the tip of the screw.
It is known from U.S. Pat. No. 4,484,570 to expand the head of the screw to provide secure fixation of the screw to the plate. However, this suffers from the disadvantage that a force arises from the tapered mating of the inner and outer screws which may contribute to loosening and subsequent screw pull out leading to failure of the device.
According to the present invention there is provided a surgical implant comprising a plate having at least two apertures and two screw threaded fastening devices for securing the plate to bone, each aperture in the plate having an annular groove intermediate opposite ends of the aperture and each fastening device comprising an outer screw and an inner plug, the screw having a head at one end, a tip at the other end, an externally threaded shank between the head and the tip and an internal bore extending axially through the head and at least part way into the shank for receiving the inner plug, the head comprising a plurality of resiliently deformable fingers separated from one another by axially extending slits, each finger having an outwardly projecting rib extending circumferentially of the head intermediate opposite ends of the finger for snap fit engagement in the annular groove of a respective plate aperture, and the inner plug being arranged so that when it is inserted into the internal bore of the screw it will prevent contraction of the head of the screw thereby preventing the ribs disengaging from the groove.
The invention will now be more particularly described, by way of example, with reference to the accompanying drawings.